Electronic utility meters include circuits that obtain power from a power supply that is connected to the utility power lines. In the event of a power interruption on the power lines, electronic utility meters typically shut down or enter a “sleep” mode due to the lack of power to the power supply. Conventionally, electronic meters employ a series of shutdown operations upon detection of a power interruption to ensure a graceful power down. The shutdown operations include the storage of various metering values into flash memory or the like. In addition, if the meter includes an automatic meter reading (“AMR”) transmitter, then the shutdown operations can include a so-called “last gasp” transmission that transmits a trouble signal and/or metering information to a remote device monitored by the utility service provider.
Because the utility power to the power supply has been lost in a power interruption, the shutdown operations obtain power from one or more temporary source of power, such a capacitor or a battery. In order to ensure that a sufficient amount of temporary power is available, it is important to initiate the last gasp transmission and other shut down procedures in a timely manner. Without a timely detection of this event, normal operations may continue, depleting the energy stored in the temporary power supply, compromising the meter's ability to complete a last gasp communication.
There are known methods of detecting a power outage. For example, it is known to use a DC-rectified measurement of the line voltage to determine when line voltage is interrupted. Such circuits typically involve a voltage divider, a diode and a smoothing capacitor. The circuit elements are arranged such that when nominal voltage (e.g. 240 VAC) is available, the output signal has a voltage level above a threshold voltage for signaling to processing circuitry that line voltage is present. One drawback of this method is that in order to accommodate line voltage fluctuations, the threshold must be set to allow for lower average line voltages. As a result, when a line voltage interruption occurs when the line voltage is normal, the circuit has a delayed reaction to a line interruption due to the delay in the decline of the average voltage past the threshold set for lower average line voltages.
Accordingly, another known method of providing line voltage presence information is using a zero crossing detector. In particular, electricity meters often include zero-crossing detectors that are used to help synchronize internal oscillator circuits. Such zero-crossing detectors provide a digital output signal that transitions each time the detected line voltage crosses zero volts in the AC cycle. The constant 60 Hz digital output signal thus provides both a synchronization reference signal for internal timing circuits, but also a “heartbeat” signal used by the processing circuit to determine whether line voltage is present. Specifically, if the processing circuit identifies that that the “heartbeat” signal missed a transition, then it can begin power down operations.
A recent development in electricity meters is the three-phase power supply. In the past, meters employed a power supply for internal circuitry powered off of one of the three line phase voltages. For example, in a well-known three-phase power system including phase A, phase B and phase C, the electricity meter power supply may be connected to phase C. A drawback to this configuration was that it is possible that power could be lost on phase C, but not phase A and/or phase B. In such a case, the meter power supply would lose input power, leading to shutdown of the meter, even though the customer or load was still receiving power on phase A and phase B. Accordingly, it has become more common to employ a three-phase power supply that provides power to the internal metering circuits of an electricity meter as long as there is power on any of the three phases.
Meters employing three-phase power supplies require a power outage detection circuit that can detect when line voltage is missing from all three phases of the electrical service. If and only if the meter loses power on all three phases, then the meter can perform the power down operations. In one prior art example, the meters used a composite signal of three zero-crossing pulse signals. Each phase included a zero-crossing circuit that generated a short-duration pulse signal at each zero crossing. The short-duration pulse signals of all three phases were combined into a single pulse signal. The resulting pulse signal had a varying frequency and/or duty cycle based on the presence or absence of certain phases of the circuit. For example, if power was available on all three phases, then the composite pulse signal would have a pulse frequency of 360 pulses per second (2 zero crossings per cycle, 60 cycles per second, for each of three phases). If, however one of the phases was missing, then the composite pulse signal would have a pulse frequency of 240 pulses per second.
While the composite signal allowed for the processing circuit to detect the presence of at least one line voltage, the use of the signal for this purpose and for synchronization purposes was complex, given the variable frequency and duty cycle.
Accordingly, there exists a need for a method and apparatus for early detecting of power outages in all phases of a multiple phase power line that provides for simpler processing and use for synchronization purposes.